Interleukin 17 (IL-17 or CTLA-8), a cytokine secreted by Th17 cells, is profoundly associated with inflammatory diseases, autoimmune diseases, and infectious diseases. Human IL-17 is a 20-30 kDa glycoprotein configured with 155 amino acids, comprising a signal peptide at the N-end. In the molecular structure thereof, six cysteine residues and one N-binding sugar chain binding site are present. The mature form consists of 136 amino acids, normally occurring as a dimer.
As proteins of the IL-17 family, six kinds of proteins are known: IL-17A, B, C, D, E, and F. Generally, IL-17 refers to IL-17A. IL-17E is also called IL-25. The amino acid sequence homology of human IL-17 to human IL-17B, C, D, E, and F is 25, 28, 22, 27, and 44%, respectively, IL-17F being of the highest homology. Human IL-17 has homologies of 63% and 90% to mouse IL-17 and marmoset IL-17. As receptors thereof, IL-17RA, IL-17RB, IL-17RC, IL-17RD, and IL-17RE are known. IL-17 and IL-17F form a homodimer or heterodimer and binds to IL-17RA and IL-17RC. The binding of IL-17 and IL-17RA is weak at a Kd value of about 10−7, suggesting that the involvement of IL-17RC may be important.
The Th17 cells are CD4+ T cells that produce IL-17. When memory CD4+. T cells are stimulated with IL-23 in vitro, IL-17 production is induced. Meanwhile, TGF-β and IL-6 play an important role in the differentiation induction of Th17 cells. TGF-β and IL-6 act on naive T cells to induce the expression of RORgt (transcriptional factor). Because a deficiency in RORgt makes Th17 cells to be unable to differentiate, and also because naive T cells can conversely be differentiated into IL-17-producing cells by forcibly expressing RORgt, this transcriptional factor is thought to be important to the differentiation of Th17 cells. Although activation of STAT3 by IL-6 is important to the induction of the expression of RORgt, activation of STAT5 by IL-2 conversely suppresses the expression. IL-2 is necessary for the differentiation of regulatory T cells; IL-2-deficient mice experience serious autoimmunity; this is thought to be due to a decrease in regulatory T cells and concurrent over-differentiation of Th17 cells. When naive T cells are stimulated with TGF-β alone in vitro, not Th17, but regulatory T cells, are induced. IFN-γ produced by Th1 cells, IL-4 produced by Th2 cells, and the like work suppressively on the differentiation of Th17 cells.
When IL-17 binds to a receptor, the NF-κB pathway, MAP kinase pathway, and C/EBP pathway are activated via Act-1 and TRAF6, resulting in the induction of inflammatory cytokines and chemokines. For example, IL-17 acts on macrophages to induce the expression of IL-1, TNF, MMP-9 and the like. In addition, IL-17 is known to act also on connective tissue system cells such as fibroblasts and endothelial cells, and on immune system cells such as dendritic cell progenitor cells, to induce the expression of various cytokines and receptors such as IL-6, IL-1, and ICAM-1.
Involved in the production of IL-17 are cytokines such as TNF-α, IL-1β, IL-6, and IFN-γ. Meanwhile, production of these cytokines is induced by IL-17. IL-17 is known to act synergistically with other cytokines.
IL-17 has been found to be profoundly associated with inflammatory diseases, autoimmune diseases and the like. It is known that the expression of IL-17 is elevated in patients with chronic rheumatoid arthritis, systemic lupus erythematosus, Behçet's disease, graft rejection, nephritic syndrome, inflammatory bowel disease, asthma, multiple sclerosis, periodontal disease and the like. In IL-17-deficient mice, it has been reported that collagen-induced arthritis (CIA), which is a model of chronic rheumatoid arthritis; experimental autoimmune encephalomyelitis (EAE), which is a model of multiple sclerosis; contact type hypersensitivity reactions by DNFB or TNCB; delayed type hypersensitivity reactions by methylated BSA; airway hypersensitive reactions by OVA induction, and the like are remarkably suppressed.
IL-17 is also associated with cancers. It has been reported that subcutaneous transplantation of non-small cell lung cancer cells to SCID mice promotes the proliferation of cancer cells in mice having IL-17 expressed highly therein. It has also been reported that IL-17 is also associated with uterine cervical cancer and ovarian cancer.
IL-17 is associated with infectious diseases. IL-17R-knockout mice are highly susceptible to Klebsiella pneumoniae infection, Candida albicans infection, Toxsoplasma gondii infection and the like. IL-17 production is induced by lipopolysaccharides (LPS) and bacterial cell body components such as of Borrelia burgdorferi and Klebsiella pneumoniae. These components are thought to promote IL-17 production by acting on antigen-presenting cells to induce IL-23. In IL-17R-knockout mice, after Klebsiella pneumoniae infection, in infected sites in the lung, the production of CXCL1, CXCL2, G-CSF and the like, which play an important role in the migration and functions of neutrophils, has been reduced, with a disturbance noted in the migration of neutrophils.
In recent years, applications of RNA aptamers to therapeutic drugs, diagnostic reagents, and test reagents have been drawing attention; some RNA aptamers have already been in clinical study stage or in practical use. In December 2004, the world's first RNA aptamer drug, Macugen, was approved as a therapeutic drug for age-related macular degeneration in the US. An RNA aptamer refers to an RNA that binds specifically to a target molecule such as a protein, and can be prepared using the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) method (International Patent Publication WO91/19813, WO94/08050, WO95/07364). In the SELEX method, an RNA that binds specifically to a target molecule is selected from an RNA pool with about 1014 different nucleotide sequences. The RNA used has a random sequence of about 40 residues, which is flanked by primer sequences. This RNA pool is allowed to mix with a target molecule, and only the RNA that has bound to the target molecule is collected using a filter and the like. The RNA collected is amplified by RT-PCR, and this is used as a template for the next round. By repeating this operation about 10 times, an RNA aptamer that binds specifically to the target molecule can be acquired.